Effects of Nonadditive Genetic Interactions, Inbreeding, and Recessive
Defects on Embryo and Fetal Loss by Seventy Days

Lethal recessive genes that cause early embryo loss are
difficultto detect. Nonreturn rate at 70 d after first insemination
(NR)was evaluated as a trait of the embryo using 1,739,055
first-servicerecords from 1,251 Holstein bulls represented as both
servicesires and sires of cows. Effects modeled included
herd-year-season,parity of cow, sire of cow, service bull,
interaction of servicebull with sire of cow, and regression on
inbreeding of embryo.Variances of service bull and sire of cow were
estimated usingREML and estimated effects were removed from the
data. Interactionvariance was estimated from the residuals using the
tilde-hatapproximation to REML. An additive relationship matrix was
usedfor sire of cow and a dominance relationship matrix for theinteraction term. Service bull effects were assumed constantacross time and unrelated. For each 10% increase in inbreeding,NR percentage declined by an estimated 1%. A regression of thissize could be explained by > 20% of animals carrying defectsthat cause early embryo loss. Of the total variance, servicebull contributed 0.36%; sire of cow, 0.24% (heritability of1.0%); and interaction, 0.18% (dominance variance of 2.8%).Numbers of records exceeded 500 for 50 bull pair subclasses.Predicted interactions that included effects of inbreeding rangedfrom -3.6% to +2.9%, compared with the mean NR of 56%.The
largest negative interactions were not caused by known recessivedefects. Complex vertebral malformation generally causes lossof pregnancies later in gestation, and few current bulls carrythe gene for deficiency of uridine monophosphate synthase. Furtherstudy of the families with largest negative interactions coulduncover new recessive defects.